First Fossil Frog from Antarctica: Implications for Eocene High
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www.nature.com/scientificreports OPEN First fossil frog from Antarctica: implications for Eocene high latitude climate conditions and Gondwanan cosmopolitanism of Australobatrachia Thomas Mörs1,2*, Marcelo Reguero3 & Davit Vasilyan4,5 Cenozoic ectothermic continental tetrapods (amphibians and reptiles) have not been documented previously from Antarctica, in contrast to all other continents. Here we report a fossil ilium and an ornamented skull bone that can be attributed to the Recent, South American, anuran family Calyptocephalellidae or helmeted frogs, representing the frst modern amphibian found in Antarctica. The two bone fragments were recovered in Eocene, approximately 40 million years old, sediments on Seymour Island, Antarctic Peninsula. The record of hyperossifed calyptocephalellid frogs outside South America supports Gondwanan cosmopolitanism of the anuran clade Australobatrachia. Our results demonstrate that Eocene freshwater ecosystems in Antarctica provided habitats favourable for ectothermic vertebrates (with mean annual precipitation ≥900 mm, coldest month mean temperature ≥3.75 °C, and warmest month mean temperature ≥13.79 °C), at a time when there were at least ephemeral ice sheets existing on the highlands within the interior of the continent. Consistent with the geological evidence, it has been hypothesized that the formation of Antarctic ice sheets predates the fnal break-up of Gondwana, the opening of the Drake Passage and the thermal isolation of the continent1–3. Tis is refected by a low diversity of terrestrial mammals on the Antarctic Peninsula during the middle to late Eocene with only two species of large mammals and ten species of small mammals4–6 which sharply contrasts with the highly diverse marine fsh fauna indicating temperate conditions in the Weddell Sea7,8. However, no Cenozoic ectothermic continental vertebrates (freshwater fshes, amphibians and reptiles) have been known from Antarctica so far. Here we report the discovery of a fossil ilium from Seymour Island, Antarctic Peninsula (Fig. 1a,b) which can be assigned to the lissamphibian order Anura, and a fragment of a sculptured skull bone that most probably derived from a hyperossified anuran. We assign the specimens to the South American genus Calyptocephalella. Calyptocephalellids, or helmeted frogs, are widely known from Patagonia since the Late Cretaceous9. Tey became extinct in Argentine Patagonia during the Miocene, probably related to a decrease of humidity caused by the rise of the Andes, since the family survived to the present day in a temperate but humid refuge in the central Chilean Andes10. Te material described here derives from estuarine to marginal-marine deposits of the Eocene La Meseta Formation which were deposited in the James Ross Basin, a back-arc basin east of the Antarctic Peninsula, and which are widely exposed on Seymour Island11,12 (Fig. 1b,c). Te fossil locality IAA 2/95, also known as ‘Marsupial Site’ is a few m2 wide lens of poorly consolidated, shelly conglomerate8,13. It is situated in the central portion of the Cucullaea I Allomember, within unit Telm 5 on the northwestern slope of the mesa (Fig. 1c,d), and informally referred to as the ‘Natica horizon’8,13. It has produced shark, ray and skate teeth, remains of marine bony fshes, as well as teeth of terrestrial mammals, worm (clitellate) cocoons, and seeds of water lilies8,12–22. Based on dinocyst occurrences, the age of this deposit is considered to be about 40 Ma (Bartonian, Eocene)23,24. 1Department of Palaeobiology, Swedish Museum of Natural History, P.O. Box 50007, SE-104 05, Stockholm, Sweden. 2Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden. 3Instituto Antártico Argentino, Campus Miguelete, 25 de Mayo 1151, 3° piso B1650HMK, San Martín, Buenos Aires, Argentina. 4JURASSICA Museum, Route de Fontenais 21, 2900, Porrentruy, Switzerland. 5Department of Geosciences, University of Fribourg, Chemin du musée 6, 1700, Fribourg, Switzerland. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:5051 | https://doi.org/10.1038/s41598-020-61973-5 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Maps and stratigraphic column of the studied area and succession. (a) Map of Antarctica showing the location of the study area. (b) Map of the northern Antarctic Peninsula showing the location of Seymour Island. (c) Geological sketch map of Seymour Island, showing the position of the fossil locality (asterisk). (d) Stratigraphic column of the La Meseta Formation on Seymour Island (from11) showing the position of the fossil locality (asterisk). Redrawn from13. Te fossil frog remains were collected during three joint Argentinian-Swedish expeditions to Seymour Island in the austral summers 2011–13. Te bone fragments were concentrated from dry-sieved sediment samples as described by8,12,20 and sorted by using a Leica MZ6 stereomicroscope. Te material is housed in the palaeozoo- logical collections of the Swedish Museum of Natural History, Stockholm, with the inventory numbers NRM-PZ B281 and B282. Results Systematic palaeontology. Anura Fischer von Waldheim, 181325 Neobatrachia Reig, 195826 Australobatrachia Frost et al., 200627 Calyptocephalellidae Reig, 196028 Calyptocephalella Strand, 192829 Calyptocephalella sp. Figures 2 and 3 Referred specimens. Swedish Museum of Natural History NRM-PZ B282, right ilium (Fig. 2), NRM-PZ B281, skull bone (Fig. 3). Locality, horizon and age. IAA 2/95, Marsupial site, Seymour Island, Antarctic Peninsula (64°13′58″S; 56°39′06″W). ‘Natica horizon’ within the Cucullaea I Allomember (Telm 5) of the La Meseta Formation, Bartonian (40 Ma), Eocene23,24 (Fig. 1). SCIENTIFIC REPORTS | (2020) 10:5051 | https://doi.org/10.1038/s41598-020-61973-5 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 2. Ilium (NRM-PZ B282) of Calyptocephalella sp. from Seymour Island, Antarctica. Ilium in lateral (a), medial (b), ventral (c) and dorsal (d) views. Magnifed region of the dorsal protuberance in lateral (e – dashed rectangle in orange color) and dorsal (f – dashed rectangle in yellow color) view. Te dashed line in black on (a) indicates the probable outline of the posterior extension of the ventral acetabular expansion. Te asterisk (*) on (a,c) indicates the shallow and broad depression of the ventral acetabular expansion. Te double asterisk (**) indicates the notch caudally from the dorsal protuberance. Te dashed red lines on (e,f) outline the intact bone surface. Abbreviations: ac, acetabulum; ar, acetabular rim; dae, dorsal acetabular expansion; is, iliac shaf; paf, preacetabular fossa; spf, supraacetabular fossa; vae, ventral acetabular expansion; vd, ventral depression. Measurements. Te preserved part of the ilium measures 3.9 mm in length, the distance from the tip of the dorsal acetabular expansion to the (preserved) tip of the ventral acetabular expansion measures 3.3 mm, the highest height of the acetabular fossa equals 2.5 mm. Te skull bone measures 2.7 mm at its both broadest and longest parts. Ilium. Te fragmentary right ilium (NRM-PZ B282) lacks the caudal portion of the acetabulum and most of the iliac shaf. Te dorsal acetabular expansion has a smooth lateral surface and is higher than the preserved part of the ventral acetabular expansion (Fig. 2a). A large and deep supraacetabular fossa is present at its base (Fig. 2a,d). Te preserved portion of the acetabulum is concave and its shape allows concluding a (semi-)circular outline. Te acetabular rim is most prominent at its anterior part (Fig. 2a,c). Te barely-developed ventral acetab- ular expansion projects ventrally. Te posterior-most portion of the ventral acetabular expansion is broken of. However, the anterior portion of the ventral acetabular expansion is higher than the preserved posterior portion. In ventral view (Fig. 2a,c), the lateral surface of the ventral acetabular expansion is convex. Te ventral acetabular expansion possesses a shallow and broad depression. In the preacetabular zone, a small and shallow preacetabular fossa is present (Fig. 2c). Te preserved portion of the iliac shaf is damaged and precludes a confdent statement whether the dorsal protuberance is present or absent. A narrow and shallow longitudinal groove is observa- ble in the lateral surface of the iliac shaf, which probably corresponds to the posterior extension of the ventral SCIENTIFIC REPORTS | (2020) 10:5051 | https://doi.org/10.1038/s41598-020-61973-5 3 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 3. Skull bone fragment (NRM-PZ B281) of Calyptocephalella sp. from Seymour Island, Antarctica in dorsal (a), ventral (b) and lateral (c) views. species locality coll nr. SVL (in mm) HT (in mm) RHS (in %) Reference C. pichileufensis Río Pichileufú locality BAR 85b 107 6 5.6 48: Figs. 2 and 4 C. gayi unknown M13105/cas:sua:10082* 54.7 2.9 5.3 65, see Fig. S3 range 5.3–5.6 mean ± SD 5.45 ± 0.21 C. sp. IAA 2/95, Marsupial site NRM-PZ B282 38.4 ± 3.8 2.1 Present work Table 1. Measurements of the snout-vent length (SVL) and height of the transition from the iliac shaf and ilial body (HT) of some Calyptocephalella spp. and Antarctic ilia (Fig. S3), with the value of the ratio between HT and SVL (RHS = HT/SVL*100%) with the value of the standard deviation (SD). Te reconstructed value of the snout-vent length of the studied ilium is highlighted in bold. depression (sensu10) (Fig. 2a). However, intact parts of bone surface are preserved slightly ventral to the dorsal margin on both lateral and medial surfaces (Fig. 2e,f). Te one on the lateral surface is a curved shallow groove and runs posteroventrally (Fig. 2e). Tis feature anteroventrally demarcates the slightly elevated roughened scar interpreted above as the dorsal protuberance. Te area between the dorsal acetabular expansion and iliac shaf is slightly projected dorsally. Tis area corresponds to the position of the dorsal protuberance. In fact, no clear evi- dence of a dorsal protuberance can be found on the ilium, only a slightly roughened area with minimal elevation that corresponds to the dorsal protuberance and the scar for the insertion of the musculus gluteus magnus can be observed.